SLM's are transducers that modulate incident light in a spatial pattern pursuant to an electrical or other input. The incident light may be modulated in phase, intensity, polarization or direction. SLM's of the deformable mirror class include micromechanical arrays of electronically addressable mirror elements or pixels which are selectively movable or deformable. Each mirror element is movable in response to an electrical input to an integrated addressing circuit formed monolithically with the addressable mirror elements in a common substrate. Incident light is modulated in direction and/or phase by reflection from each element.
As set forth in greater detail in commonly assigned U.S. Pat. No. 5,061,049 to Hornbeck, deformable mirror SLM's are often referred to as DMD's (for "Deformable Mirror Device" or "Digital Micromirror Device") and are manufactured by Texas Instruments of Dallas Texas. There are three general categories of deformable mirror SLM's: elastomeric, membrane and beam. The latter category includes torsion beam DMD's, cantilever beam DMD's and flexure beam DMD's.
Each movable mirror element of all three types of beam DMD includes a relatively thick metal reflector supported in a normal, undeflected position by an integral, relatively thin metal beam. In the normal position, the reflector is spaced from a substrate-supported, underlying control electrode which may have a voltage selectively impressed thereon by the addressing circuit.
When the control electrode carries an appropriate voltage, the reflector is electrostatically attracted thereto and moves or is deflected out of the normal position toward the control electrode and the substrate. Such movement or deflection of the reflector causes deformation of its supporting beam storing therein potential energy which tends to return the reflector to its normal position when the control electrode is de-energized. The deformation of a cantilever beam comprises bending about an axis normal to the beam's axis; that of a torsion beam comprises deformation by twisting about an axis parallel to the beam's axis; that of a flexure beam, which is a relatively long cantilever beam connected to the reflector by a relatively short torsion beam, comprises both types of deformation, permitting the reflector to move in piston-like fashion. Thus, the movement or deflection of the reflector of a cantilever or torsion beam DMD is rotational with some parts of the reflector rotating toward the substrate; other parts of the reflector rotate away from the substrate if the axis of rotation is other than at an edge or terminus of the reflector. The movement or deflection of the reflector of a flexure beam DMD maintains all points on the reflector generally parallel with the substrate.
When the reflector of a beam DMD is operated in binary fashion by its addressing circuit, it occupies one of two positions, the first being the normal position which is set by the undeformed beam, the second position being a deflected position. In one of the positions, the reflector reflects incident light to a selected site, such as a viewing screen, the drum of a xerographic printer or other photoreceptor. In the other position, incident light is not reflected to the photoreceptor, and may be reflected to a light absorber.
A typical DMD includes an array of numerous pixels, the reflectors of each of which are selectively positioned to reflect or not reflect light to a desired site.
Because a potential difference must exist between the reflector and the control electrode to deflect the reflector, it is undesirable for these two elements to engage. Engagement of a deflected reflector and its control electrode effects current flow therethrough which may weld them together and/or cause the thinner beam to melt or fuse. In either event the functionality of the involved pixel is destroyed. In response to the foregoing problem, a landing electrode may be associated with each reflector. Typically, in the case of a cantilever- or torsion-beam DMD, the landing electrode resides on the substrate at a greater distance from the rotational axis than the control electrode, both distances being taken parallel to the reflector in its normal position. In a flexure-beam DMD, the top of the landing electrode may be elevated above the top of the control electrode. In view of the foregoing, the deflected reflector ultimately engages the landing electrode, but not the control electrode. To prevent damage to the reflector, the landing electrode is maintained at the same potential as the reflector. Again, see commonly assigned U.S. Pat. No. 5,061,049.
Notwithstanding the use of a landing electrode, it has been found that a deflected reflector will sometimes stick or adhere to its landing electrode. Such sticking or adherence may prevent the energy stored in the deformed beam from returning or "resetting" the reflector to its normal position after the control electrode is de-energized. It has been postulated that such sticking is caused by welding or intermolecular attraction between the reflector and the landing electrode or by high surface energy substances sorbed or deposited on the surface of the landing electrode and/or on the portion of the reflector which contacts the landing electrode. Substances which may impart high surface energy to the reflector-landing electrode interface include water vapor and other ambient gases (e.g., carbon monoxide, carbon dioxide, oxygen, nitrogen), and gases and organic components resulting from or left behind following production of the DMD, including gases produced by outgassing from UV-cured adhesives which mount a protective cover to the DMD. A protective cover and other DMD "package" is disclosed in commonly assigned U.S. Pat. No. 5,293,511 entitled "Package for a Semiconductor Device", the teachings of which are incorporated herein by reference.
Sticking of the reflector to the landing electrode has been overcome by applying selected numbers, durations, shapes and magnitudes of voltage pulses ("reset signals") to the control electrode. One type of reset signal attempts to further attract a reflector toward its landing electrode, which the reflector already engages. This further attraction stores additional potential energy in the already deformed beam. When the control electrode is de-energized, the increased potential energy stored in the beam is now able to unstick the reflector from the landing electrode and return the reflector to its normal position. A variant reset signal comprises a train of pulses applied to the control electrode to induce a resonant mechanical wave in a reflector already engaging a landing electrode. De-energizing the control electrode as a portion of the reflector is deformed away from the landing electrode unsticks the reflector. For more details concerning the foregoing and other unsticking techniques, see commonly assigned U.S. Pat. No. 5,096,279 and co-pending patent application Ser. No. 08/311,480, entitled "Manufacturing Method for Micromechanical Devices" filed Sep. 23, 1994, the teachings incorporated herein by reference.
In commonly assigned U.S. Pat. No. 5,331,454 entitled "Low Reset Process for DMD", there is disclosed a technique for passivating or lubricating the portion of the landing electrode engaged by the deformed reflector, and/or the portion of the deformed reflector which engages the landing electrode, so that sticking or adherence therebetween is reduced or eliminated. Passivation is effected by lowering the surface energy of the landing electrode and/or the reflector, which is, in turn, effected by chemically vapor-depositing on the engageable surfaces of interest a monolayer of a long-chain aliphatic halogenated polar compound, such as a perfluoroalkyl acid. This acid is characterized by a chain having an F.sub.3 C molecule at a first end, a COOH molecule at the second end, and intermediate CF.sub.2 molecules. The COOH end becomes firmly attached to surfaces of the DMD--following pretreatment of such surfaces, if necessary, to achieve same--to present the very low surface energy F.sub.3 C and CF.sub.2 molecules for engagement. The application of such a compound to at least that portion of the landing electrode which is engaged by a deformed reflector has resulted in an amelioration of the sticking or adhesion problem.
Objects do not easily, if at all, stick or adhere to low energy surfaces, which are also usually expected to be resistant to sorption thereonto of the above-discussed high-surface-energy-imparting substances, such as water vapor. However, while DMD's on which the above-described anti-stick monolayer has been deposited initially exhibit little, if any, reflector-electrode adherence--as evidenced by the low magnitudes of reset signals--after time, higher magnitudes of reset signals are required. Similarly, when protective, light-transparent covers are mounted to DMD's with adhesives, such as UV-cured epoxies, a need to increase the magnitude of reset signals over time has been noted. A similar effect has also been noted in DMD's after several hours of "burn-in." The foregoing suggests that substances--in the first case from the ambient, in the second case outgassed from the adhesive, in the third case outgassed from the DMD--are adhering to or becoming incorporated into the low surface energy anti-stick deposit, possibly due to defects or discontinuities in the films (or coatings) thereof.
Elimination of the sticking phenomenon described above over the lifetime of the device is an object of the present invention.